[Haskelldb-users] errors evaluating aggregates

[Rafael J. Fernández-M. <rfe...@cs...>]

Hello everybody!

I am getting a runtime error when I evaluate aggregates. For example, 
consider:

query1 = do s <- table sailors
            project (sid <<  _sum (s!sid))

"sid" is an attribute in table "sailors", of type Int (and NOT NULL). 
When evaluating, I get a runtime error:

user error (The type of sid field can't be converted to SqlUnknown 246 type)

My intuition is that the type of a sum expression is not necessarily 
Int, since you can always sum Doubles. Well, the next obvious step for 
me was to rewrite the query and make the "receiving" column type a double:

data Doublefield = Doublefield

instance FieldTag Doublefield where fieldName _ = "doublefield"

doublefield ::  Attr Doublefield Double
doublefield = mkAttr Doublefield

query2 = do s <- table sailors
            project (doublefield <<  _sum (s!sid))


Oops, that won't compile, the inferred type of sum is indeed Int (or so 
I'm told by GHC):

    Couldn't match expected type `Double' against inferred type `Int'
      Expected type: Expr Double
      Inferred type: Expr Int

Strangely, count, max, and min do work. I wonder if (1) I'm missing 
something, or (2) this is indeed an unexpected behaviour (given query1 
passes the typechecker) that may be specific to MySQL...

Thanks,

RJFM